Thermal imaging device

ABSTRACT

A thermal imaging device comprising collector means to focus infra-red rays, a fluorescent screen enclosed in a sealed evacuated capsule and including a phosphor on a membrane, said fluorescent screen being positioned at the focal plane of the collector means, an energizing source for the fluorescent screen and means for varying the fluorescence which results from the presence of an infra-red image on the screen.

United States Patent [1 1 Jones et al.

THERMAL IMAGING DEVICE Inventors: Neal K. Jones, Brighton, SouthAustralia; Douglas W. Faulkner, Salisbury Heights, South Australia;Robert T. Johnston, North Walkerville, South Australia; Murray R.Meharry, Elizabeth Park, South Australia, all of Australia Assignee: TheCommonwealth of Australia,

Melbourne, Victoria, Australia Filed: May 1, 1967 Appl. No.1 637,036

US. Cl 250/333, 250/334, 250/365,

313/92 Int. Cl. 1101 39/00 Field of Search 250/833 IR, 71,

[ Dec. 11, 1973 [56] References Cited UNITED STATES PATENTS 2,482,8159/1949 Urbach 250/833 lR 3,015,731 l/l962 Van Santen et al. 250/833 1R3,365,576 1/1968 Teeg 250/833 [R 3,370,172 2/1968 Hora 250/833 1RPrimary Examiner-Benjamin A. Borchelt Assistant Examiner-H. A. BirmielAttorney-Waters, Roditi, Schwartz & Nissen [57] ABSTRACT A thermalimaging device comprising collector means to focus infra-red rays, afluorescent screen enclosed in a sealed evacuated capsule and includinga phosphor on a membrane, said fluorescent screen being positioned atthe focal plane of the collector means, an energizing source for thefluorescent screen and means for varying the fluorescence which resultsfrom the presence of an infra-red image on the screen.

30 Claims, 8 Drawing Figures THERMAL IMAGING DEVICE This inventionrelates to an improved thermal imaging device and in particular itrelates to a device of the type which has as its object the collectionof infra-red rays from a heat source near ambient temperature and therendering visible of these rays to an operator.

The principle of utilising infra-red rays to produce an image is alreadyknown and such images are produced on a screen where they can be vieweddirectly, or by an optical magnification device, or alternatively theycan be produced by a scanning device which permits sensitivity to beincreased by electrical means.

Certain problems exist with this type of apparatus which it is theobject of this invention to overcome, the chief problem being to obtainhigh resolution and sensitivity with rapidity of image production sothat a picture displaying moving objects can be viewed by the operatorwith a minimum amount of relay and with sufficient definition andcontrast.

It will be realised of course that temperature sensitive screens arestill in early stages of development so far as resolution is concernedbut apart from the screens themselves there are certain problems whichrequire to be solved for practical purposes such as the reduction of theapparatus to a relatively small size for the particular purposeconcerned, and a reduction of auxiliary gear which should be avoidedwhere such a unit is suitable for field work.

The unit can actually be used for various purposes but one of the primepurposes is to provide a means for fire detection, or the detection ofhot or warm spots, particularly where the source of the heat is obscuredby smoke or haze or the like as is usually the case in fires.

According to an earlier patent application of ours a fire centredetector was provided which also used infrared rays from a heat sourcebut in that case the mirror was so designed that it directed theincoming rays to a bolometer through a chopper and in that case apicture could only be obtained by traversing the unit and noting thefluctuations of the signal resulting therefrom, the object of thepresent invention of course being to provide a picture covering arequired area all of which is visible at the same time to the viewer.

The objects of the invention are achieved by providing a unit comprisingan optical collecting system for the infra-red rays and a temperaturesensitive, light emitting screen in the path of such collected rays,means being provided to energise the screen and also to view the screen,to emit visible light such as a binocular viewer or scanning systemusing electrical means to amplify and display the picture signal.

A feature of the invention which, though not essential but is preferred,is a self-contained or capsulated detector unit or cell requiring noelaborate external apparatus and not requiring to be cooled with liquidnitrogen or like means to hold correct conditions within the unit.

Thus the invention in its basic form comprises collector means to focusinfra-red rays, a fluorescent screen at the focal plane of the collectormeans, an energising source for the said fluorescent screen, and meansto adjust the screen temperature to its optimum operating temperature,independently of ambient temperature, the screen comprising a supportfilm with an absorber film thereon for infra-red rays, and on saidabsorber film phosphor particles in heat conductive contact with theabsorber film, but with low thermal transfer between phosphor particleswhereby the infra-red radiation produces a temperature pattern on thescreen which in turn varies the fluorescence of the screen.

According to one form of the invention it comprises a housing which hastowards its rear a primary aspheric mirror which is apertured at thecentre to provide viewing through it by a binocular viewer or the like,which housing also carries a secondary aspheric mirror towards the frontthereof and of a size sufficiently small to give the minimum obstructionof the field of the pri mary aspheric mirror, the two mirrors being setto direct the infra-red rays on to a detector disposed forwardly of theprimary mirror, said detector containing a fluorescent screen which iscapable of modification by an infra-red image and which is provided withenergising means in the form of a lamp or electron beam to fluoresce thescreen, and which may have, but does not necessarily have, a biastemperature control to maintain the temperature sensitive screen withinit at the correct working conditions, the arrangement of the unit beingsuch that infra-red rays striking the primary aspheric mirror arefocused by the secondary mirror on to the axially disposed temperaturesensitive screen through a suitable window which passes the infra-redrays to the screen, which screen is itself energised to fluorescence butis modulated by the incoming infrared light which in the case ofphosphors reduces fluorescence and which modified image is visible tothe viewer through the aperture in the primary mirror, thus enabling theviewer to see an actual image on the screen consistent with the heatvalues of the area covered by the collection mirrors.

As screens of this type present problems in preventing thermal spreadwith consequent loss of definition and to insure greatest possiblesensitivity, a capsulated detector unit or cell is proposed which iscompletely sealed and evacuated and which need only have a fluorescingsource applied to it apart from the infra-red image, thereby avoidingexternal vacuum applying means and consequent complication.

So that the invention can be fully understood an embodiment will now bedescribed in more detail with reference to the accompanying drawings butit is to be clear that the invention need not necessarily be limited tothis, the scope being defined in the claims forming part of thisspecification.

In the drawings:

FIG. 1 is a sectional perspective view of one form of the invention,

FIG. 2 is a schematic view of the invention,

FIG. 3 is a section of the capsule,

FIG. 4 shows a modified unit,

FIGS. 5, 6 and 7 are schematic views showing how the sensitive phosphorfilm can be formed, and

FIG. 8 shows phosphor brightness-temperature curves.

The outer housing 1 is of somewhat cylindrical form with a window 2 atthe front through which the infrared rays can enter the housing, and amirror 3 towards the rear of the housing which is the primary asphericmirror collecting the infra-red rays and concentrating them on to asecondary aspheric mirror 4 near the front of the housing 1 which inturn directs the rays into the detector cell 5 which is disposedcoaxially within the housing I and is supported by a mechanismprojecting through an aperture 6 in the mirror 3 which allows axialmovement of the supporting means 7 for the detector cell to take placeunder control of the operator, this movement being for the purpose offocusing the image on to the screen 8 in the detector cell 5.

Pockets 9 and 10 around the outside of the housing 1 carry theelectronics 11 and batteries 12.

The detector cell 5 comprises a sealed housing 15 which is shaped togive a minimum interception of the infra-red rays from the mirror systemand has at its forward end a lens 16 which may be made of potassiumbromide or other material capable of efficiently transmitting infra-redradiation, and which together with the primary and secondary asphericmirrors 3 and 4 constitutes an infra-red optical system with requiredminimum aberrations, field of view, and flat image plane, and throughwhich the infra-red radiation enters the cell 5, and immediately behindthis infra-red transmitting lens a temperature sensitive screen 17, thisscreen being arranged to glow when activated by an ultraviolet radiationsource 18 such as a mercury vapour discharge lamp, but the glow beingmodified by the infra-red rays striking the screen 8.

The ultraviolet radiation source 18 is provided with a lens system 19which directs the ultraviolet rays on to a multi-layer beam splitter 20which is designed to reflect a very high percentage of the ultravioletrays to the temperature sensitive screen 8 but allow good lighttransmission between the temperature sensitive screen 8 and the opticalviewing system 21.

An ultraviolet filter 19a may be included.

The rear of the detector unit is sealed by a window 22 of fused silicaor other material transparent to ultraviolet and visible radiation whichpreferably is arranged to be not quite normal to the axis of the unit soas to reduce reflection, and immediately behind this is the multi-layerbeam splitter 20 which receives the rays from the ultraviolet radiationsource 18 by means of which the phosphor screen 8 is energised, themultilayer beam splitter 20 of course being of such a nature thatbinocular or similar viewing mechanisms behind it allows the imagescreen to be viewed in an unobstructed manner.

Also within the capsule or cell so formed is a heater element 23 bymeans of which a thermal bias can be applied to the temperaturesensitive screen 8, this preferably being in the nature of a grid whichcan be heated by passing an electric current through the grid so thatthe whole of the face of the temperature sensitive screen can bemaintained at a required temperature for best results or for a requiredlower sensitivity.

In the modification shown in FIG. 4 the housing 30 has at its front ametal honey-comb 31 to form a protective window for the front of theunit but to allow infra-red rays to reach the mirror 32 at the rear ofthe housing, the detector cell 33 being disposed within the housing toreceive infra-red rays through the potassium bromide lens 34, acorrector lens 35 being disposed between the honey-comb 31 and themirror 32.

In this case the energising and viewing of the screen of the detectorcell 33 is from the back of the unit and an ocular viewing arrangementis shown at 36 while the lamp supplying the ultraviolet light isdisposed behind the mirror 32 but the rays are reflected by themultilayer beam splitter 38 through the lens system 39 to the detectorcell 33, the visual rays from the phosphor screen of the detector cell33 passing through the multi-layer beam splitter 38 and focusing in theocular 36.

It will be realised that this is simply a rearrangement of the mechanismdescribed with reference to the foregoing figures and it will also berealised that in both of these cases it is possible to modify the systemfor flying spot scanning or for television scanning where amplificationof the signal is required.

In the case of a flying spot scanner the ultraviolet light source wouldbe replaced by a flying spot scanner which would direct the electronbeam onto the screen for scanning purposes.

In the case of the television scanner the eye-piece would be replaced bymeans of the television scanner so that an image of the screen wouldthen be available for amplification by any required amount in the normaltype of television system and also this would permit transmission of thepicture so obtained by normal techniques.

To ensure that the temperature sensitive screen 8 will operate underbest conditions the detector cell 15 is evacuated for the purpose ofminimising heat transfer on the temperature sensitive screen itself andloss of heat from the screen to the surroundings, for it will berealised that, as the screen is actuated by thermal differentials, thesethermal differentials must be maintained by the avoidance of heattransfer in the screen 8 itself and to any surroundings.

The actual construction of the screen can be varied but according to aconvenient form it comprises a phosphor, which phosphor does not itselfabsorb the infrared radiation, used in conjunction with an absorberwhich rises in temperature under the action of infra-red radiation andraises the temperature of adjacent phosphor particles by thermalconduction, and in this regard it can be mentioned that the phosphorscreen can be of the monolayer type or of a segmented type to avoidlateral conduction or thermal spread, the absorber itself being eithercoated on the phosphor particles or on to the very thin, lowconductivity base on which the particles are carried, or the said verythin, low conductivity supporting film for the phosphor particles canitself embody the absorber, the above conditions requiring thin filmssuch as single layer phosphor layers to preserve the correct temperatureisolation but another system can be used in which groups of absorber andphosphor layers are provided in the nature of a grid giving thenecessary thermal isolation to the sections of the grid without havingto use the thin layer of phosphor.

The production of the temperature sensitive screen is critical butaccording to one form, as illustrated in FIG. 5, a collodion supportfilm 35 which is about 750 Angstrom thick was prepared by allowing adrop or two of a 6 percent solution (weight per volume) of collodion inamyl acetate to spread over a water surface, and then lifting thehardened film on a ring. The phosphor particles 26, having a maximumeffective diameter of about 2 microns, were then settled on to the filmfrom a suspension in petroleum ether with lead oleate as a dispersingagent. The absorbing film 37 was lead oleate as a dispersing agent. Theabsorbing film 37 was then deposited by vacuum evaporation, thethickness being adjusted by monitoring the transmission at a selectedwavelength.

In this way the phosphor particles adhere to the film by naturalattraction, but sufficiently to give good adhesion unless physicallybrushed off. By depositing the phosphor particles from a liquid,particularly with a dispersing agent which provides a stable suspensionof the particles in the liquid, the support deposition of the particlesis such that the particles tend to remain separated and thermal transferfrom particle to particle is minimised.

Another method of forming the support film, which in some cases may bean improvement on the first stated method in that it gives films havingbetter thickness uniformity across the individual films and from batchto batch, consists in lowering a clean glass disc completely into. a 1.3percent collodion solution and withdrawing at a constant slow rate.After being allowed to dry the film is removed from the glass plate bybeing placed at a slight angle from the horizontal in a dish which isslowly and steadily filled with water. The film rises to the surface ofthe water and is lifted off on a ring as before. Precoating of the platewith a release layer of alkali halide improves the stripping process.

In the case where the absorber such as gold black particles is to beembedded in the support film, see FIG. 6, the absorber particles 40 werefirst deposited on a glass disc and the second above process was thenapplied, the absorber particles being then entirely enclosed within thesupport film 41, thermal conduction between the absorber and thephosphor particles 42 being then assured by the collodion film, thisleading to high sensitivity.

Using segmentedscreens, see FIG. 7, certain advantages result in thatthermal transfer can only occur through the supporting membrane 45 atareas such as 46 where no phosphor particles 47 are deposited and wherethe absorber film 48 is absent, consequently high resolution is obtainedwith small segments. The phosphor particles do not have to be depositedas a monolayer with spacing between the individual particles to avoidthermal transfer, and thus there can be a denser layer. Provided all thephospor particles come to the temperature of the absorber there will bean increase in average screen luminance without loss of sensitivity.

, Also the greater number of particles in the picture element givegreater uniformity over the area of the screen and from screen toscreen. Deposition of the absorber film can be through a grid, andxerographic processes can then be used to deposit the phosphorelectrostatically according to the grid pattern.

In FIG. 8 is shown a graph of phosphor brightness to temperature, fromwhich it will be seen that by selecting the temperature of the phosphoras controlled by the heater 23, optimum results are obtainable when thecorrect temperature of the phosphor is maintained.

By using the detector unit 8 in the form of a completely sealed capsuleor cell it will be obvious that the unit does not have to be suppliedwith vacuum lines to lower the pressure within same which is necessaryto prevent thermal transfer within the capsule itself, and thus a simpleand effective unit results which can be battery operated so far as heator bias is concerned and also the mercury vapour discharge lamp which isthe energising source for the phosphor can similarly be actuated from abattery supply, thus making the unit completely self-contained andallowing it to traverse an area where inspection is to be made such asfor fire detection purposes or the like, the viewing screen according tothis invention ensuring good resolution and a very effective type ofcontrol because of the variable heat or bias and also because ofeffective control of the amount of energisation of the screen from thefluorescent lamp. It will be appreciated however that if it waspreferred not to use a sealed detector unit then a vacuum drawing nipplecould be included, and the device would then have to be provided withvacuum drawing means to maintain optimum working condition of the unit.In the sealed unit the usual vacuum drawing con ditions may obtain usinga getter if required so that a unit is assured of having no morecomplications'after manufacture than a thermionic valve and readilyreplaceable under field conditions.

What we claim is:

1. An improved thermal imaging device comprising a housing, an evacuatedsealed capsule in said housing, said housing having a window foradmission of infra-red radiation therein, collector means within saidhousing for focussing the admitted infra-red rays at a focal plane ofthe collector means, said capsule including a temperature sensitivescreen therein disposed at said focal plane, said screen comprising afilm with an absorber for infra-red rays and phosphor particles on saidfilm in heat conductive contact with the absorber but with low thermaltransfer between phosphor particles, energizing means in said housingforcausing fluorescence of said phosphor particles and means for detectingvariation in fluorescence on said screen due to the presence thereat ofan infra-red image, said collector means comprising at least oneaspherical mirror arranged to direct infra-red radiation to said screen,an optical viewing system for said screen, and a beam splitterpositioned to pass light to said optical viewer system, said energizingmeans comprising an ultraviolet ray generator directed towards the beamsplitter to be focussed thereby onto said screen.

2. An improved thermal imaging device according to claim 1 comprising athermal heating grid in said capsule adjacent said screen and means toenergize the said grid.

3. An improved thermal imaging device according to claim 1 wherein themeans for detecting variation in fluorescence on said screen comprises atelevision scanner focussed on the said screen.

4. An improved thermal imaging device according to claim 1 wherein saidcollector means comprises a correcting lens, said aspherical mirrorbeing arranged behind said correcting lens to direct infra-red rays fromthe window to said capsule.

5. An improved thermal imaging device according to claim 1 wherein thephosphor particles on the screen are in the form of a mono-layer.

6. An improved thermal imaging device according to claim 1 comprisingbias temperature control means for the said screen to control brightnessof the screen.

7. An improved thermal imaging device according to claim 1, wherein saidcapsule includes a grid adapted to be heated to raise uniformly thetemperature of the phosphor particles for brightness control.

8. An improvided thermal imaging device according to claim 1, whereinsaid screen is segmented both for the phosphor particles and theabsorber film to prevent lateral spread of heat through the screen.

9. An improved thermal imaging device according to claim 1, wherein saidabsorber film is coated on one side of the said support film and thephosphor particles are on the other side of the support film.

10. An improved thermal imaging device according to claim 1, wherein theabsorber film is coated onto the phosphor particles.

11. An improved thermal imaging device comprising a housing, anevacuated sealed capsule in said housing, said housing having a windowfor admission of infra-red radiation therein, collector means withinsaid housing for focussing the admitted infra-red rays at a focal planeof the collector means, said capsule including a temperature sensitivescreen therein disposed at said focal plane, said screen comprising afilm with an absorber for infra-red rays and phosphor particles on saidfilm in heat conductive contact with the absorber but with low thermaltransfer between phosphor particles, energizing means in said housingfor causing fluorescence of said phosphor particles and means fordetecting variation in fluorescence on said screen due to the presencethereat of an infra-red image, the said absorber film being aparticulate substance embedded in the said support film.

12. An improved thermal imaging device according to claim '11 whereinsaid collector means comprises at least one aspherical mirror arrangedto direct infra-red rays to said capsule, said energizing meanscomprising ultraviolet ray producing means focussed onto the saidphosphor screen.

13. An improved thermal imaging device comprising a housing, anevacuated sealed capsule in said housing, said housing having a windowfor admission of infra-red radiation therein, collector means withinsaid housing for focussing the admitted infra-red rays at a focal planeof the collector means, said capsule including a temperature sensitivescreen therein disposed at said focal plane, said screen comprising afilm with an absorber for infra-red rays and phosphor particles on saidfilm in heat conductive contact with the absorber but with low thermaltransfer between phosphor particles, energizing means in said housingfor causing fluorescence of said phosphor particles and means fordetecting variation in fluorescence on said screen due to the presencethereat of an infra-red image, said collector means comprising at leastone aspherical mirror arranged to direct infra-red radiation to saidscreen, an optical viewing system for said screen and a beam splitterpositioned to pass light to said optical viewer system, said energizingmeans comprising a flying spot scanner directed towards the beamsplitter to be focussed thereby onto said screen.

14. An improved thermal imaging device comprising a housing, anevacuated sealed capsule in said housing, said housing having a windowfor admission of infra-red radiation therein, collector means withinsaid housing for focussing the admitted infra-red rays at a focal planeof the collector means, said capsule including a temperature sensitivescreen therein disposed at said focal plane, said screen comprising afilm with an absorber for infra-red rays and phosphor particles on saidfilm in heat conductive contact with the absorber but with low thermaltransfer between phosphor particles, energizing means in said housingfor causing fluorescence of said phosphor particles and means fordetecting variation in fluorescence on said screen due to the presencethereat of an infra-red image, said collector means comprising at leastone aspherical mirror arranged to direct infra-red rays to said capsule,said energizing means comprising ultraviolet ray producing meansfocussed onto the said phosphor screen, and heater means within thecapsule to raise uniformly the temperature of the phosphor particles.

15. An improved thermal imaging device comprising collector means tofocus infra-red radiation, a fluorescent screen at the focal plane ofthe collector means, said screen comprising a support film with anabsorber thereon for infra-red radiation, and phosphor particles on saidfilm in heat conductive contact with the absorber on the film but withlow thermal transfer between phosphor particles, an energizing sourcefor said fluorescent screen, the infra-red radiation producing atemperature pattern on the screen which in turn varies the fluorescenceof the screen, and means for detecting the variation of fluorescence dueto the presence of the infra-red radiation on said screen.

16. An improved thermal imaging device according to claim 15 comprisinga sealed evacuated capsule enclosing said fluorescent screen.

17. An improved thermal imaging device according to claim 16 whereinsaid capsule includes a grid adapted to be heated to raise uniformly thetemperature of the phosphor particles for brightness control.

18. An improved thermal imaging device according to claim 16 whereinsaid collector means comprises at least one aspherical mirror arrangedto direct infra-red rays to said capsule, said energizing meanscomprising ultraviolet ray producing means focussed onto the saidphosphor screen.

19. An improved thermal imaging device according to claim 16 comprisingheater means within the capsule to raise uniformly the temperature ofthe phosphor particles.

20. An improved thermal imaging device according to claim 16 comprisinga thermal heating grid in said capsule adjacent said screen and means toenergize the said grid.

21. An improved thermal imaging device according to claim 15 whereinsaid collector means comprises at least one aspherical mirror arrangedto direct infra-red radiation to said screen, an optical viewing systemfor said screen, and a beam splitter positioned to pass light to saidoptical viewer system, said energizing means comprising an ultravioletray generator directed towards the beam splitter to be focussed therebyonto said screen.

22. An improved thermal imaging device according to claim 21 whereinsaid collector means comprises a correcting lens, said aspherical mirrorbeing arranged behind said correcting lens to direct infra-red rays fromthe window to said capsule.

23. An improved thermal imaging device according to claim 15 whereinsaid collector means comprises at least one aspherical mirror arrangedto direct infra-red radiation to said screen, an optical viewing systemfor said screen and a beam splitter positioned to pass light to saidoptical viewer system, said energizing means comprising a flying spotscanner directed towards the beam splitter to be focussed thereby ontosaid screen.

24. An improved thermal imaging device according to claim 15 wherein themeans for detecting variation in fluorescence on said screen comprises atelevision scanner focussed on the said screen.

25. An improved thermal imaging device according to claim 15 wherein thephosphor particles on the screen are in the form of a mono-layer.

26. For an improved thermal imaging device, an evacuated sealed capsulecontaining a temperature sensitive screen, said capsule having a windowto admit infra-red radiation, said screen comprising a support film withan absorber film thereon for infra-red radiation and phosphor particleson said support film in heat conductive contact with the absorber filmbut with low thermal transfer between phosphor particles, and a heateradjacent said support film for uniformly raising the temperature of thephosphor particles for optimum effect.

27. An improved thermal imaging device according to claim 26 whereinsaid screen is segmented both for the phosphor particles and theabsorber film to prevent lateral spread of heat through the screen.

substance embedded in the said support film.

1. An improved thermal imaging device comprising a housing, an evacuatedsealed capsule in said housing, said housing having a window foradmission of infra-red radiation therein, collector means within saidhousing for focussing the admitted infra-red rays at a focal plane ofthe collector means, said capsule including a temperature sensitivescreen therein disposed at said focal plane, said screen comprising afilm with an absorber for infra-red rays and phosphor particles on saidfilm in heat conductive contact with the absorber but with low thermaltransfer between phosphor particles, energizing means in said housingfor causing fluorescence of said phosphor particles and means fordetecting variation in fluorescence on said screen due to the presencethereat of an infra-red image, said collector means comprising at leastone aspherical mirror arranged to direct infra-reD radiation to saidscreen, an optical viewing system for said screen, and a beam splitterpositioned to pass light to said optical viewer system, said energizingmeans comprising an ultraviolet ray generator directed towards the beamsplitter to be focussed thereby onto said screen.
 2. An improved thermalimaging device according to claim 1 comprising a thermal heating grid insaid capsule adjacent said screen and means to energize the said grid.3. An improved thermal imaging device according to claim 1 wherein themeans for detecting variation in fluorescence on said screen comprises atelevision scanner focussed on the said screen.
 4. An improved thermalimaging device according to claim 1 wherein said collector meanscomprises a correcting lens, said aspherical mirror being arrangedbehind said correcting lens to direct infra-red rays from the window tosaid capsule.
 5. An improved thermal imaging device according to claim 1wherein the phosphor particles on the screen are in the form of amono-layer.
 6. An improved thermal imaging device according to claim 1comprising bias temperature control means for the said screen to controlbrightness of the screen.
 7. An improved thermal imaging deviceaccording to claim 1, wherein said capsule includes a grid adapted to beheated to raise uniformly the temperature of the phosphor particles forbrightness control.
 8. An improvided thermal imaging device according toclaim 1, wherein said screen is segmented both for the phosphorparticles and the absorber film to prevent lateral spread of heatthrough the screen.
 9. An improved thermal imaging device according toclaim 1, wherein said absorber film is coated on one side of the saidsupport film and the phosphor particles are on the other side of thesupport film.
 10. An improved thermal imaging device according to claim1, wherein the absorber film is coated onto the phosphor particles. 11.An improved thermal imaging device comprising a housing, an evacuatedsealed capsule in said housing, said housing having a window foradmission of infra-red radiation therein, collector means within saidhousing for focussing the admitted infra-red rays at a focal plane ofthe collector means, said capsule including a temperature sensitivescreen therein disposed at said focal plane, said screen comprising afilm with an absorber for infra-red rays and phosphor particles on saidfilm in heat conductive contact with the absorber but with low thermaltransfer between phosphor particles, energizing means in said housingfor causing fluorescence of said phosphor particles and means fordetecting variation in fluorescence on said screen due to the presencethereat of an infra-red image, the said absorber film being aparticulate substance embedded in the said support film.
 12. An improvedthermal imaging device according to claim 11 wherein said collectormeans comprises at least one aspherical mirror arranged to directinfra-red rays to said capsule, said energizing means comprisingultraviolet ray producing means focussed onto the said phosphor screen.13. An improved thermal imaging device comprising a housing, anevacuated sealed capsule in said housing, said housing having a windowfor admission of infra-red radiation therein, collector means withinsaid housing for focussing the admitted infra-red rays at a focal planeof the collector means, said capsule including a temperature sensitivescreen therein disposed at said focal plane, said screen comprising afilm with an absorber for infra-red rays and phosphor particles on saidfilm in heat conductive contact with the absorber but with low thermaltransfer between phosphor particles, energizing means in said housingfor causing fluorescence of said phosphor particles and means fordetecting variation in fluorescence on said screen due to the presencethereat of an infra-red image, said collector means comprising at leastone aspherical mirror arranged to direct infra-red radiation to saidscreen, aN optical viewing system for said screen and a beam splitterpositioned to pass light to said optical viewer system, said energizingmeans comprising a flying spot scanner directed towards the beamsplitter to be focussed thereby onto said screen.
 14. An improvedthermal imaging device comprising a housing, an evacuated sealed capsulein said housing, said housing having a window for admission of infra-redradiation therein, collector means within said housing for focussing theadmitted infra-red rays at a focal plane of the collector means, saidcapsule including a temperature sensitive screen therein disposed atsaid focal plane, said screen comprising a film with an absorber forinfra-red rays and phosphor particles on said film in heat conductivecontact with the absorber but with low thermal transfer between phosphorparticles, energizing means in said housing for causing fluorescence ofsaid phosphor particles and means for detecting variation influorescence on said screen due to the presence thereat of an infra-redimage, said collector means comprising at least one aspherical mirrorarranged to direct infra-red rays to said capsule, said energizing meanscomprising ultraviolet ray producing means focussed onto the saidphosphor screen, and heater means within the capsule to raise uniformlythe temperature of the phosphor particles.
 15. An improved thermalimaging device comprising collector means to focus infra-red radiation,a fluorescent screen at the focal plane of the collector means, saidscreen comprising a support film with an absorber thereon for infra-redradiation, and phosphor particles on said film in heat conductivecontact with the absorber on the film but with low thermal transferbetween phosphor particles, an energizing source for said fluorescentscreen, the infra-red radiation producing a temperature pattern on thescreen which in turn varies the fluorescence of the screen, and meansfor detecting the variation of fluorescence due to the presence of theinfra-red radiation on said screen.
 16. An improved thermal imagingdevice according to claim 15 comprising a sealed evacuated capsuleenclosing said fluorescent screen.
 17. An improved thermal imagingdevice according to claim 16 wherein said capsule includes a gridadapted to be heated to raise uniformly the temperature of the phosphorparticles for brightness control.
 18. An improved thermal imaging deviceaccording to claim 16 wherein said collector means comprises at leastone aspherical mirror arranged to direct infra-red rays to said capsule,said energizing means comprising ultraviolet ray producing meansfocussed onto the said phosphor screen.
 19. An improved thermal imagingdevice according to claim 16 comprising heater means within the capsuleto raise uniformly the temperature of the phosphor particles.
 20. Animproved thermal imaging device according to claim 16 comprising athermal heating grid in said capsule adjacent said screen and means toenergize the said grid.
 21. An improved thermal imaging device accordingto claim 15 wherein said collector means comprises at least oneaspherical mirror arranged to direct infra-red radiation to said screen,an optical viewing system for said screen, and a beam splitterpositioned to pass light to said optical viewer system, said energizingmeans comprising an ultraviolet ray generator directed towards the beamsplitter to be focussed thereby onto said screen.
 22. An improvedthermal imaging device according to claim 21 wherein said collectormeans comprises a correcting lens, said aspherical mirror being arrangedbehind said correcting lens to direct infra-red rays from the window tosaid capsule.
 23. An improved thermal imaging device according to claim15 wherein said collector means comprises at least one aspherical mirrorarranged to direct infra-red radiation to said screen, an opticalviewing system for said screen and a beam splitter positioned to passlight to said optical viewer system, said energizing mEans comprising aflying spot scanner directed towards the beam splitter to be focussedthereby onto said screen.
 24. An improved thermal imaging deviceaccording to claim 15 wherein the means for detecting variation influorescence on said screen comprises a television scanner focussed onthe said screen.
 25. An improved thermal imaging device according toclaim 15 wherein the phosphor particles on the screen are in the form ofa mono-layer.
 26. For an improved thermal imaging device, an evacuatedsealed capsule containing a temperature sensitive screen, said capsulehaving a window to admit infra-red radiation, said screen comprising asupport film with an absorber film thereon for infra-red radiation andphosphor particles on said support film in heat conductive contact withthe absorber film but with low thermal transfer between phosphorparticles, and a heater adjacent said support film for uniformly raisingthe temperature of the phosphor particles for optimum effect.
 27. Animproved thermal imaging device according to claim 26 wherein saidscreen is segmented both for the phosphor particles and the absorberfilm to prevent lateral spread of heat through the screen.
 28. Animproved thermal imaging device according to claim 26 wherein saidabsorber film is coated on one side of the said support film and thephosphor particles are on the other side of the support film.
 29. Animproved thermal imaging device according to claim 26 wherein theabsorber film is coated onto the phosphor particles.
 30. An improvedthermal imaging device according to claim 26 wherein the absorber filmis a particulate substance embedded in the said support film.